Printed wiring members are commonly used as ways to electrically interconnect electronic components in low cost fashion. Some types of printed wiring members are designed for direct interconnection to semiconductor chips by wire bonding or tape automated bonding, for example. Furthermore, some types of printed wiring members are designed to have connector pads that are intended to disconnectably mate multiple times with an electrical connector that connects the printed wiring member with other circuitry in an electronic system. An example of a printed wiring member that both directly interconnects to a semiconductor chip and also has disconnectable connection pads is an inkjet printhead. In this example, the semiconductor chip is the inkjet printhead die, which typically contains the nozzles, the drop forming mechanisms to eject drops from the nozzles, and electronics associated with the drop forming mechanisms. Because printheads typically do not last the entire lifetime of the printer, many types of printheads are designed to be disconnectable from the printer to allow replacement.
Although nominally a printhead would only need to be installed and uninstalled once, it is preferable to design the printhead connector pads to withstand more than 10-20 installation cycles in order to improve reliability of the system. One well-known way of making connector pads reliably connectable for many cycles is to provide a top metallization of hard gold on the connector pads. Hard gold is made hard during the plating process by adding cobalt and/or nickel at levels of approximately 0.1% to 0.3%, although higher levels of impurities can be incorporated. As is well-known in the art, however, hard gold is not readily wire-bondable. In order to provide a printed wiring member with bondable gold at the contact pads and wear resistant gold at the connector pads, one approach would be to do two separate masking and plating steps to provide soft gold (around 99.9% pure) at the contact pads and hard gold at the connector pads, but this is relatively costly.
U.S. Pat. No. 5,910,644 discloses metallization electroplated onto the copper contact pads and connector pads of a printed wiring member having both bondability and wear resistance. The disclosed metallization includes 80-200 microinches (2-5 microns) of nickel plated onto the copper, nominally 35 microinches (0.9 micron) of palladium plated onto the nickel, and 5-30 microinches (0.1-0.75 micron) of soft gold plated onto the palladium. The high purity soft gold provides for high yield wire bonding while the palladium ensures adequate wear resistance to provide stable electrical connection in the event that wear through of the soft gold surface finish of the connector pads occurs. A drawback of this process is that relatively thick layers of the costly palladium and high purity gold are required.
U.S. patent application Ser. Nos. 12/627,133 and 12/627,161 respectively disclose a printed wiring member and a method for making a printed wiring member that has wire bondable contact pads and wear-resistant connector pads. In particular, it is disclosed that a palladium layer is electrolessly deposited over a layer of high purity soft gold. The palladium is sufficiently bondable for the contact pads, and also provides additional wear resistance for the connector pads. The resulting structure can be connected and disconnected more than 20 times without excessive wear for at least some connector designs. However, if the connector produces an aggressive wiping motion across the connector pads, or if the connector pins are somewhat rough, achieving more than 20 connects and disconnects without excessive wear can be marginal.
Consequently, a need exists for a method of making a printed wiring member that provides reliable bondability and improved wear-resistant connector pads for repetitive printhead installations. Although an inkjet printhead is a familiar example of such a need, there are other examples in the chip-on-board industry of the need for a method of making a printed wiring member that provides reliable bondability and wear-resistant connector pads for repetitive component installations.